To conclude this confused discussion, we shall have to lay down a principle of definition which will help to determine the essential differences and resemblances that exist either between the parts of a same subject, or between its faculties, or between its parts and its faculties. This will clearly reveal whether in the organism the soul really has several parts, or merely several faculties, and what opinion about them should be adopted. (For there are two special types of these.) The one attributes to man a single soul, genuinely composed of several parts, either by itself, or in relation to the body. The other one sees in man a union of several souls, looking on the man as on a choir, the harmony of whose parts constitutes its unity, so that we find several essentially different parts contributing to the formation of a single being. First we shall have to study within the soul the differentials between the part, the faculty and the disposition. A part always differs from another by the substrate, genus, and function. A disposition in a special aptitude of some one part to carry out the part assigned to it by nature. A faculty is the habit of a disposition, the power inherent in some part to do the thing for which it has a disposition. There was no great inconvenience in confusing faculty and disposition; but there is an essential difference between part and faculty. Whatever the number of faculties, they can exist within a single "being," or nature, without occupying any particular point in the extension of the substrate, while the parts somewhat participate in its extension, occupying therein a particular point. Thus all the properties of an apple are gathered within a single substrate, but the different parts that compose it are separate from each other. The notion of a part implies the idea of quantity in respect to the totality of the subject. On the contrary, the notion of a faculty implies the idea of totality. That is why the faculties remain indivisible, because they penetrate the whole substrate, while the parts are separate from each other because they have a quantity. How then may we say that a soul is indivisible, while having three parts? For when we hear it asserted that she contains three parts in respect to quantity, it is reasonable to ask how the soul can simultaneously be indivisible, and yet have three parts. This difficulty may be solved as follows: the soul is indivisible in so far as she is considered within her "being," and in herself; and that she has three parts in so far as she is united to a divisible body, and that she exercises her different faculties in the different parts of the body. Indeed, it is not the same faculty that resides in the head, in the breast, or in the liver;[357] (the seats of reason, of anger and appetite). Therefore, when the soul has been divided into several parts, it is in this sense that her different functions are exercised within different parts of the body.